The invention relates to center lapping tools for precisely lapping center holes of opposed ends of a workpiece in order to facilitate subsequent high precision "inside diameter outside diameter", or ID/OD, grinding of the workpiece.
It is well known to those in the machine tool business that extremely close tolerances need to be maintained during an operation known as "between-centers grinding". In "between-centers grinding" operations, a cylindrical workpiece is supported by means of "center holes" disposed in opposite ends of the workpiece. The center holes of the workpiece ordinarily originally are provided therein by means of a lathe. One end of the workpiece is positioned in the jaws of the lathe, and a center drill is used to drill one center hole in the opposed end of the workpiece. The workpiece is then removed from the jaws of the lathe chuck and reversed, and the procedure is repeated to cut the second center hole in the other end of the workpiece. Unfortunately, it is very difficult to achieve the desired accuracy in alignment of the two center holes formed in this manner. Furthermore, the roundness of such center holes is not as accurate as is sometimes needed. Cylindrical workpieces with lathe-cut center holes often are heat treated before the between-centers grinding operation, and this usually results in some warpage, which misaligns the lathe-cut center holes, even if they initially are perfectly aligned and perfectly round. In order to solve this problem, a prior art machine made by EX-CEL-O Corporation of 549 Cedar Street, Newington, Conn. 06111, known as a "center lapping machine" was devised many years ago and remains the only such machine that is commonly used. This center lapping machine includes a stationary, rigidly supported, upwardly oriented conical support, referred to herein as a "dead center" cone coaxially disposed with a rotary conical grinding stone that is commonly referred to as a "lapping stone" mounted on a motor driven spindle. This spindle is driven by a belt and pulleys, and has four speeds which can be varied only by changing a belt and pulleys. The lapping stone can be "dressed" by means of a built-in dressing tool that is pivotal by means of a gate-like hinge mechanism to swing a diamond dressing tip adjacent to the rotary lapping stone. The diamond tip point can be moved along the surface of the rotating lapping stone, trimming it precisely to the proper angle, removing any buildup of material that may have occurred. To continue center lapping operations, the dressing tool is pivoted away from the lapping stone. The user positions one end of a workpiece to be center lapped so that the tip of the dead center cone is inserted into that center hole. The operator then lowers the spindle on which the lapping stone is supported into the upper center hole, and lightly grinds the workpiece material surrounding the upper center hole, thereby producing a new, cleanly lapped, precisely oriented center hole. The workpiece is then inverted, and the same operation is repeated to lap the other center hole.
Although the EX-CEL-O center lapping machine has been the "industry standard" for many years, it suffers from a number of shortcomings. One is that the dressing tool mechanism does not always provide the extremely precise accuracy necessary to always dress the lapping stone as accurately as is needed. This problem is especially pronounced for older machines, due to the wear in the hinge mechanism and the failure of the dressing mechanism to return to precisely the same dressing position every time it is used. Other problems with this prior art center lapping machine include the necessity of a time consuming belt changing operation to attain one speed which is satisfactory for dressing a particular stone and another spindle speed which is necessary for center lapping a particular type of workpiece with that dressed lapping stone. This operation sometimes must be repeated every time the lapping stone is dressed, which may be as often as every dozen or so center lapping operations, since hard metal workpieces rapidly deteriorate the grinding surface of even the hardest lapping stone. The time required to change spindle speeds by carrying out a belt changing operation greatly reduces the efficiency of the machinists in carrying out the between centers grinding operations. In modern applications, required tolerances for between centers grinding in the aircraft turbine industry range from vary one ten-thousandth to a millionth of an inch. These tolerances cannot be maintained on a high yield basis without excess use of high cost machinist's time with the above described prior art center lapping machine. Furthermore, some lapping stones have cone angles other than the sixty degree angle, which is the only dressing angle capable of being utilized by the prior art center lapping machine.
Thus, there is an unmet need for an improved center lapping machine which is capable of consistently producing extremely high precision, precisely aligned center holes in workpieces to allow extremely precise between centers grinding operations to be performed on the workpieces.
Accordingly, it is an object of the invention to provide an improved center lapping machine and method for providing extremely precise dressing of lapping stones with an extremely high degree of repeatability.
It is another object of the invention to provide an improved center lapping machine which avoids excessive inconvenience and waste of time by the operator in changing spindle speeds from an optimum dressing speed to an optimum center lapping speed.
It is another object of the invention to provide an improved center lapping machine and method which avoids inaccuracy in dressing of a lapping stone due to failure of the dressing mechanism to always return to precisely the same position for each dressing operation.
It is another object of the invention to provide an improved center lapping machine that avoids inaccuracy in the center lapping operation due to slight misalignment of the dead center cone.
It is another object of the invention to provide an improved center lapping machine which precisely and conveniently can dress a plurality of lapping stones having different cone angles.